Well I purchased a set a rear Timbrens for my truck. After one week of testing, I decided to return them, here is why. Extremely easy install, that's a given. They absolutely minimized the rock...
As others have noted, if you adjust them wrong, they can be bad. I went with airbags because they are easier to adjust. Of course, air bags can be bad as well which is why people go away from airbags to timbrens. :)